1. Technical Field
The present invention relates to a power-off detection device and, more particularly, to a power-off detection device that enables an electronic appliance to detect the power-off of external input power from a power supply device that supplies DC power to the electronic appliance so as to safely prepare for the power-off.
2. Related Art
A typical power supply device for an electronic appliance provides power by converting AC (Alternating Current) power into DC (Direct Current) power.
The power-off detection device detects a power-off of the external input power that provides the AC power to this typical power supply device, and detects the power-off of the external input power at a certain time before the supply of the DC voltage to the electronic appliance is stopped, so that the central control means of the electronic appliance can complete its work safely without any loss.
The typical power-off detection circuit comprises a number of resistors, a bridge rectifier, a photo coupler, a capacitor and a voltage comparator, and the photo coupler includes a photodiode and a phototransistor.
For this power-off detection circuit, the AC power inputted as external input power from a power supply device is transmitted and rectified via the resistor and the bridge rectifier.
Further, the transmitted and rectified power is applied to the photodiode of the photo coupler, and the photodiode emits light depending on the applied AC power.
Further, the phototransistor of the photo coupler converts the emitted light of the photodiode into a corresponding electrical signal.
The photodiode and the phototransistor of the photo coupler should be kept dielectric for electrical stability between circuits.
Further, the capacitor is charged by a predetermined current of the electrical signal produced by the phototransistor, and the capacitor rectifies the charged current to produce a DC voltage.
When the predetermined current is not provided by the phototransistor, the capacitor discharges through a non-inverting (+) terminal of the voltage comparator, thereby providing a comparison voltage thereto.
A reference voltage is applied to the inverting (−) terminal of the voltage comparator according to the resistance ratio between voltage dividing resistors while the a comparison voltage is applied by the capacitor to the non-inverting (+) terminal of the voltage comparator.
Therefore, the voltage comparator compares the reference voltage with the comparison voltage, and when there is a difference between the two voltages, a power-off detection signal is outputted to an output terminal through a pull-up resistor during a predetermined time period.
That is, when the comparison voltage value is not equal to the established reference voltage in the voltage comparator, a predetermined power-off detection signal is outputted through a pull-up resistor.
The output terminal of the voltage comparator, which is the output terminal of the power-off detection circuit, is connected to central control means of the electronic appliance, and when the power-off detection signal is received from the output terminal of the power-off detection circuit, the central control means completes its work safely while primary power continues to power-off.
In preparation for the power-off of the primary power, the central control means can store, in storage means, information as to currently running or pending work, and can complete its work safely before the supply of DC power ceases.
This typical power-off detection circuit has to use the photo coupler having the photodiode and the phototransistor to keep dielectric relationship between circuits.
However, there is a problem in that the cost of manufacturing the power supply device increases because the photo coupler is expensive.
Further, when designing the power supply device, if a photo coupler is employed, it is difficult to guarantee a safe distance between a primary circuit and a secondary circuit, and to make the appliance smaller since the area occupied by the photo coupler becomes large.
The following patents are considered to be generally pertinent to the present invention, but are burdened by many of the disadvantages set forth above: U.S. Pat. No. 6,378,068 to Foster et al., entitled SUSPEND/RESUME CAPABILITY FOR A PROTECTED MODE MICROPROCESSOR, issued on Apr. 23, 2002; U.S. Pat. No. 6,223,293 to Foster et al., entitled SUSPEND/RESUME CAPABILITY FOR A PROTECTED MODE MICROPROCESSOR, issued on Apr. 24, 2001; U.S. Pat. No. 6,081,752 to Benson IV et al., entitled COMPUTER SYSTEM HAVING POWER SUPPLY PRIMARY SENSE TO FACILITATE PERFORMANCE OF TASKS AT POWER OFF, issued on Jun. 27, 2000; U.S. Pat. No. 5,765,004 to Foster et al., entitled SUSPEND/RESUME CAPABILITY FOR A PROTECTED MODE MICROPROCESSOR, issued on Jun. 9, 1998; U.S. Pat. No. 5,603,038 to Crump et al., entitled AUTOMATIC RESTORATION OF USER OPTIONS AFTER POWER LOSS, issued on Feb. 11, 1997; U.S. Pat. No. 5,560,023 to Crump et al., entitled AUTOMATIC BACKUP SYSTEM FOR ADVANCED POWER MANAGEMENT, issued on Sep. 24, 1996; U.S. Pat. No. 5,551,043 to Crump et al., entitled STANDBY CHECKPOINT TO PREVENT DATA LOSS, issued on Aug. 27, 1996; U.S. Pat. No. 5,511,204 to Crump et al., entitled PERFORMING SYSTEM TASKS AT POWER-OFF USING SYSTEM MANAGEMENT INTERRUPT, issued on Apr. 23, 1996; U.S. Pat. No. 5,175,847 to Mellott, entitled COMPUTER SYSTEM CAPABLE OF PROGRAM EXECUTION RECOVERY, issued on Dec. 29, 1992; U.S. Pat. No. 4,641,064 to Testin et al., entitled TELEVISION POWER SUPPLY SHUTDOWN CIRCUIT, issued on Feb. 3, 1987; U.S. Pat. No. 5,903,766 to Walker et al., entitled SUSPEND/RESUME CAPABILITY FOR A PROTECTED MODE MICROPROCESSOR, issued on May 11, 1999; U.S. Pat. No. 5,758,174 to Crump et al., entitled COMPUTER SYSTEM HAVING A PLURALITY OF STORED SYSTEM CAPABILITY STATES FROM WHICH TO RESUME, issued on May 26, 1998; U.S. Pat. No. 5,446,904 to Belt et al., entitled SUSPEND/RESUME CAPABILITY FOR A PROTECTED MODE MICROPROCESSOR, issued on Aug. 29, 1995; and U.S. Pat. No. 4,868,729 to Suzuki, entitled POWER SUPPLY UNIT, issued on Sep. 19, 1989.